1. Field of the Invention
The present invention relates to an inverter circuit of the type comprising a DC voltage source which itself has a center-tap or which has a center-tap in a voltage divider circuit connected across the source, and two serially connected thyristor switches which are in the forward direction connected across the DC voltage source and each of which has an RC protection connected across the thyristor and a commutation diode antiparallel with the thyristor, and a reactor connected in series with the thyristor, in which case a center branch which includes at least one reactor branches out from a point between the thyristor switches to the said center-tap.
2. Description of the Prior Art
Ever since the advent on the market of thyristors or silicon controlled rectifiers as components they have been used quite extensively in inverter circuits. The present invention relates in particular to an inverter circuit in which thyristors are used in conjunction with saturable reactors. It is therefore appropriate to note that the use of saturable magnetic components in rectifier and inverter technology is not new in itself. Saturable reactors are used commonly for protecting valves (diodes, thyristors and Hg-valves) from excessive rates of change in current and voltage, as shown in the detailed description below. Premagnetized reactors and magnetic amplifiers have also been used in rectifier and inverter technology for decades. Reference is made here to the accompanying list of publications.
______________________________________ List of References ______________________________________ /1/ U.S. Pat. No. 3,324,380 /2/ U.S. Pat. No. 3,321,695 /3/ SE Lay-Open Print 316,862 /4/ SE Patent 227,486 /5/ GB Patent 1,407,594 /6/ GB Patent 1,360,112 /7/ CH Patent 475,669 /8/ CH Patent 467,549 /9/ U.S. Pat. No. 2,635,222 /10/ U.S. Pat. No. 2,857,563 /11/ Seeling, T. Thyristorwechselrichter mit sattigbaren Drosseln fur Mittelfrequenzanwendungen IEEE Trans. on Magnetics, Vol. MAG-2, No. 3, September 1966 /12/ Min. B. J. An Improved Snubber Circuit for Wearsch, H. W. Power Semicoductors IAS 77, Annual /13/ Meyer, Manfred Beanspruchung von Thyristoren in selbstgefuhrten Stromrichtern Siemens Zeitschrift Mai 1965, Heft 5 /14/ Technische Mitteilungen 4, 5 9 AEG . ______________________________________
For example, reference /14/ discloses the use of saturable reactors for reducing the turn-on and turn-off losses of thyristors. The same subject is also dealt with in references /5/, /8/ and /13/. The use of saturable reactors for voltage increase rate protection is dealt with in, for example, references /2/, /5/ and /12/.
Saturable reactors have further been used for equalizing the turn-on delays of thyristors connected in parallel /4/ and also in series connections for the equalization of voltage division when thyristors turn on /7/.
Reference /11/ discloses a method, substantially in accordance with the Morgan principle, for increasing the effective frequency of a resonance inverter, and references /1/ and /10/ disclose applications of magnetic amplifiers.
The circuit according to accompanying FIG. 1 is discussed below; it is made up of batteries P.sub.1 and P.sub.2, thryistors T.sub.1 and T.sub.2, protecting coils L.sub.k, RC protections R.sub.1 C.sub.1, commutation capacitor C, main inductance L.sub.p, leakage inductance L.sub..sigma., as well as load R.sub.L and commutation diodes D1 and D2. The purpose of the protecting coils L.sub.k is to protect the thryistors T.sub.1 and T.sub.2 from excessive rates of current increase and, together with the RC protections, from an excessive rate of voltage increase. L.sub.p together with C forms the actual commutation circuit, the purpose of which is to turn off the thyristor, as known from the literature (Bedford-Hoft: Principles of Inverter Circuits).
When the frequency is low (less than 1 kHz), L.sub.k C.sub.1 R.sub.1 can easily be dimensioned so that, when nowadays generally available thyristors are used and E is 500-600 V (a 380 V voltage rectified with a three-stage bridge), the allowed dV/dt and dI/dt values of the thyristor will not be surpassed and, on the other hand, the losses of the RC protections remain moderate. When the frequency increases, the coils L.sub.p, L.sub..sigma. and L.sub.k and the capacitor C must be decreased. As a result, the dI/dt and dV/dt of the thyristors increase if R.sub.1 and C.sub.1 remain constant since, as known, ##EQU1## As evident from (1), the problem can be corrected by decreasing R.sub.1, but as a result C.sub.1 must be increased in order that the circuit 2L.sub.k R.sub.1 C.sub.1 remain sufficiently attenuated and not cause voltage spikes. If a critically attenuated resonance circuit is taken as the criterion, the value obtained for C is ##EQU2## i.e., the increasing of C.sub.1 follows from the decreasing of R.sub.1. Since the power loss of the RC protections is of the form EQU P.sub.loss .about.E.sup.2 f C.sub.1 ( 3)
the power loss increases quite rapidly when the frequency increases. The absolute value of the power loss is strongly dependent on the requirements set and on the components available. In the example case presented below, the tolerable limit in practice is below 5 kHz. In addition, it is advisable to keep in mind that a thrysistor will not tolerate all kinds of RC protection. Manufacturers often give an upper limit for C.sub.1 and a lower limit for R.sub.1. For this reason the frequency cannot be increased indefinitely even at the expense of losses from the RC protection.
One known method to make simultaneously possible the small coils L.sub.k required by a high frequency and the high inductances of L.sub.k required by small losses is to make the coils L.sub.k saturable. This is based on the fact that, during actual operation, the currents are so high that L.sub.k is in a saturated state and its inductance is small, thereby allowing a rather high frequency.
Instead, when the RC protections R.sub.1 C.sub.1 are operating, the current of the coil L.sub.k concerned is low, in which case the coil is in an unsaturated state and its inductance is high, in which case, on the basis of (1), dV/dt is small and, as can be seen from (1) and (2), C can be maintained small, in which case losses are small. In practice the inductance of L.sub.k in the unsaturated state can be as high as 500 times its inductance in the saturated state, and even greater changes are possible. This method has, however, a serious disadvantage, as shown below.
If it is assumed that T.sub.1 has just ceased conducting and I is commutating to D.sub.1 in a conventional manner, in this situation the voltage of U.sub.c is at its maximum and L.sub.k is in an unsaturated state, in which case its inductance is typically of the order of 100-200 .mu.H.
In this case the voltage of point A gainst B, i.e. in practice the voltage effective across T.sub.2, may rise very high since ##EQU3## When, on the basis of the above, E/2.apprxeq.300 V and the voltage U.sub.c of capacitor C, e.g..perspectiveto.800 V, are substituted ##EQU4## is obtained.
The significance of the matter increases because, when the order of the voltage of the thyristor increases, the highest speeds of the thyristors decrease, and thus, when the aim is to use high frequencies, it is necessary to use thyristors of relatively low voltages. On the other hand, connecting thyristors in series in an expensive and technically rather difficult solution.